1. Field of the Invention
The present invention generally relates to communication systems, and more particularly, to a system and method for reduced power consumption in digital subscriber line modems.
2. Discussion of the Related Art
In recent years, telephone communication systems have expanded from traditional plain old telephone system (POTS) communications to include high-speed data communications as well. As is known, POTS communications include the transmission of voice information, control signals, public switched telephone network (PSTN) information, as well as, information from ancillary equipment in analog form (i.e., computer modems and facsimile machines) that is transmitted in the POTS bandwidth.
Prompted largely by the desire of large businesses to reliably transfer information over a broadband network, telecommunications service providers have employed digital subscriber lines (DSL) to provide a plethora of interactive multi-media digital signals over the same existing POTS twisted-pair lines.
The provision of DSL services to customer premises has proliferated over recent years. DSL services are typically provided to remotely located customer premises by equipping both the appropriate central office and the customer premises with compatible DSL modems. DSL modems communicate by modulating a baseband signal with customer desired service signals, converting the modulated digital data signal to an analog signal, and transmitting the analog signal over the conventional copper wire pair provided in the PSTN from a central office to a customer premises via a carrier service area (CSA) loop. Well known data modulation and transmission techniques include mapping digital data to be transmitted into a multi-dimensional multilevel signal space constellation and decoding the received constellation to recover the transmitted information.
Once compatibly configured modems are integrated with the PSTN and xe2x80x9ctrainedxe2x80x9d to the particular response characteristics of the particular CSA local loop of the PSTN on which they communicate, they can provide a digital communications link from the customer premises to multiple digital information and communications services.
DSL communication links differ from analog or xe2x80x9cdial-upxe2x80x9d modem links in that the DSL link is designed to provide 24 hour/7 days a week network connectivity. On the other hand, xe2x80x9cdial-upxe2x80x9d modem links are more temporary in nature as they provide network connectivity upon customer demand. Full-time availability of the DSL link and the requirement for dedicated DSL equipment at the central office to service each DSL service customer result in unnecessary power consumption during those periods when the customer is not actively using the DSL link.
A major component of the total power consumption within a DSL modem is consumed by the line driver, as line driver power consumption overwhelms power consumed by digital signal processing in multiplexing, modulating, and converting the multiple DSL service input signals. As a result, output line driver power consumption is an area of concern when focusing on decreasing power consumption in DSL modems.
DSL modems use a number of different modulation schemes and rates. Asymmetric digital subscriber line (ADSL) systems adopted discrete multi-tone (DMT), a rate adaptive and tone flexible multi-carrier data transmission method. Other DSL systems use more traditional line coding methods, such as, quadrature amplitude modulation (QAM) and carrierless amplitude/phase modulation (CAP). Despite the modulation technique of choice, DSL modems are subject to time intervals when customers do not demand downstream (central office to customer premises) data transmission.
DMT systems, by nature of their distribution across multiple frequency bands, are capable of retuning modems to optimize data transfer for changing line conditions. DMT devices selectively transfer bits from the data stream in those discrete frequency bands that are uncorrupted from amplitude modulation radio interference and unaffected by phone system bridge taps, thereby tuning, or maximizing performance under changing line conditions.
Tuning of DMT system parameters is currently performed in two distinct ways: initial training, hereinafter called, xe2x80x9cfull retrain,xe2x80x9d and bit loading/swapping, an online optimization procedure. Another often suggested means to retune a system is a fast retrain of the connection. xe2x80x9cFull retrainxe2x80x9d of a DSL link results in a temporary loss of service and is undesirable under most conditions. Of the methods used to tune DMT parameters, fast retrain is best suited to overcome transient effects, while bit loading/swapping is more adapted to slowly varying changes. The fast retrain method is more robust than bit loading/swapping and provides for a more optimized system since it can actively readapt other system components such as equalizers and echo-cancelers to the actual noise environment of the communication channel.
A fast retrain algorithm may be triggered when either the central office or the remote transmission unit sense the need to transition from the current parameter profile to a more appropriate previously stored parameter profile. The most typical situation that triggers a fast retrain is when a POTS device goes on/off hook. These transitions create impedance transients that adversely affect the ADSL frequency spectra.
Fast retrain procedures are usually based upon stored profiles. It is assumed that previous full initialization procedures have been successfully completed upon earlier off-hook and on-hook transitions of POTS equipment at the customer premises. If the full initialization profile under such conditions has been stored in memory, a fast retrain can take advantage of that previous work by identifying current line conditions, recognizing if a suitable profile exists in memory, and simply recalling and applying stored profile parameters.
Before a central office DSL modem can exchange information with a remotely located DSL modem, clock timing and synchronization between the central office modem and the remote DSL modem master clock must be established. Timing and synchronization are fundamental to any digital transmission and switching network and should be maintained in order to minimize delays when resuming downstream data transmission. In a digital transmission system, timing is encoded with the transmitted signal using a network master clock. As such, the central office DSL modem must recover system timing and synchronization from this system clock. Once frequency synchronization between the central office modem and the network clock is achieved, the modem can identify frame boundaries of both the received and transmitted signals.
Accordingly, it is desired to provide a system and method that timely, efficiently, and accurately detects periods of non-use of the downstream data path in a DSL communications link, adjusts the output line driver to decrease power consumption while maintaining the necessary timing relationship between the central office and remote DSL modems to permit timely resumption of full downstream service.
Certain objects, advantages and novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the objects and advantages of the present invention, the present invention is directed to a method for detecting periods of non-use of the downstream data path in an established DSL communications link; reducing the power consumed in the output line driver of the central office modem during the period of non-use; while concurrently maintaining the necessary timing relationships between the modems. For DMT applications, a method consistent with the concepts of the present invention may also adjust the processing clock for the digital signal processor (DSP) and the analog to digital (ADC) and digital to analog converters (DAC) in the central office modem to further reduce power consumption at the central office during periods of non-use. Once the central office DSL modem has been reconfigured for reduced power operation, the method continuously searches for both upstream and or downstream signal information. Upon detecting data transmission activity in either direction, the DSL modem may perform a fast retrain to return to a full power data transfer mode.
A system consistent with the aforementioned method may be implemented with modified code and any of a number of hardware configurations capable of reducing the voltage supplied to the output line driver. This first method can be implemented by modifying code and hardware at the central office only.
Another method consistent with the concepts of the present invention introduces a xe2x80x9cstand-byxe2x80x9d processing mode that both the central office and the customer premises modems would understand. This method may be implemented by modifying code in both the central office and customer premises modems and by modifying hardware in the central office modem (to reduce the voltage supplied to the output line driver as in the first method).
The stand-by mode method introduces an additional step to the method cited above. When in stand-by mode, the central office modem transmits a reduced point constellation coded signal. In this way, the timing relationship between the two modems can be maintained and data mode can be resumed much more quickly than if the modems were required to perform a fast retrain to establish a full data mode communications link.
The methods introduced above for DSL systems that use the DMT data transmission scheme can be applied with modification to DSL systems that use CAP modulation schemes as well. In this regard, the first method may be modified to use the stand-by and xe2x80x9cwarm startxe2x80x9d functionality provided in RADSL. The second method may be modified such that in stand-by mode the central office modem is configured to transmit band edge tones in order to maintain timing recovery. The central office modem can initiate data mode simply by sending the pseudorandom noise (PN) sequence.